The present invention relates generally to amplifiers and more specifically to all forms of audio amplifiers and guitar amplifiers. It further relates to a distortion synthesizer which replicates the sounds produced by overdriven vacuum tube amplifiers.
Since semiconductor devices have become viable components for amplifiers, there has been a debate upon the virtues of semiconductor or solid state amplifiers versus the vacuum tube amplifiers. Some believe that tube amplifiers work better because vacuum tubes are more natural amplifiers than semiconductor devices. Some think that semiconductor amplifiers produce a sound that has no warmth; they are too clear until the semiconductor devices saturate, then they are too noisy. Tube amplifiers seem not to give up when overdriven, they seem to try to reach the impossible.
The vacuum tube amplifiers, however, do have some limitations. For some people, the limitation is simply the warmup time and the fragility of vacuum tubes. For guitarists the problems are more serious. A powerful amplifier does not sound right when operating at low levels such as those needed to fill a small room.
The prior art is filled with va ious attempts to satisfy the guitarists need for the tube amplifier sound with the more reliable semiconductor devices or just to enhance the sound from vacuum tube amplifiers. Moog, in U.S. Pat. No. 4,180,707, simulates the overdriven amplifier with a compressor and a clipper that can produce even harmonics as well as odd harmonics to produce the guitar sound at preamplifier power levels. Claret, in U.S. Pat. No. 4,286,492, modifies the operating point of the amplifier to clip at lower Power. Woods, in U.S. Pat. No. 3,860,876, heavily modifies the frequency response of a distorted input. Smith, in U.S. Pat. No. 4,211,893, selectively adds gain in a preamplifier stage to get distortion. Sondermeyer, in U.S. Pat. No. 4,405,832, switchable forces odd harmonic distortion and, in U.S. Pat. No. 4,439,742, creates a soft crossover distortion, also an odd harmonic phenomenon. Scholz, in U.S. Pat. No. 4,143,245, creates distortion at any sound level by operating an amplifier at maximum levels with resistive loads and driving the speaker with only a portion of the amplifier output.
In another vein, Todokoro, in U.S. Pat. No. 4,000,474, simulates a triode tube amplifier with junction field effect transistors.
The prior art is also filled with many examples of design built by many manufacturers of guitar amplifiers. Technically, the guitar amplifier is a poor example of vacuum tube amplifier design. Certainly, the guitar amplifier of today does not have the frequency response nor the clarity of the high fidelity amplifier of yesterday. However, the reason is not technical but lies in the art. The sound of inexpensive, overdriven amplifiers has become part of the art.
Thus, to simulate the vacuum tube amplifier, one must fully appreciate the nature of the components of said amplifier. One of the key components of a vacuum tube amplifier is the output transformer. The transformer passes a relatively narrow band of frequencies in the middle of the audio spectrum. The feedback in the power stage of the amplifier broadens the transformer response so that the amplifier operates effectively over a wider range of frequencies. However, when the amplifier tubes are saturated, they cannot perform the feedback function and the response narrows to the transformer response.
Of course, another key element of the vacuum tube amplifier is the tube itself. The various stages of a vacuum tube amplifier are usually coupled with capacitors that carry the signal from the output of one stage to the input of the next while blocking the constant or DC voltage of the output from the input. When the input of a stage is driven so that the grid of the vacuum tube becomes more positive than its cathode, then significant currents will flow in the grid circuit. Some of the grid current charges the coupling capacitor and thereby alters the operating point of the vacuum to amplify more asymmetrically than it usually does. When this asymmetrical waveform is amplified and clipped symmetrically by a push-pull output stage, as usually found in powerful amplifiers, it produces even as well as odd distortion harmonics. It is the even harmonics that seem to be more musical than the harsher odd harmonics.
Vacuum tubes, as all devices, has an input capacitance. The significant component of this capacitance is from the grid to the plate. This is the Miller capacitance and is multiplied by the gain of the tube. Consequentially, this input capacitance operates with grid driving impedance to limit the frequency response and to thereby change the tone of the connected musical instrument as a function of the setting of the volume control.
The gain of tubes is not constant with respect to grid voltage. The gain generally increases for increasing grid voltages. This is important when analyzing class B or AB push-pull output stages. The gain change produces harmonics in the output.
The output stage cannot use significant feedback because of the phase shifting in the transformer and other circuit components. Consequently, the output impedance of the output stage is comparatively high. This high output impedance reacts with the speaker load differently than a low output impedance. The output impedance that is associated with the proper sound is approximately that of the speaker impedance.
The output tubes of a class AB or B amplifier draw larger currents from the power supply as the input signal increases. The power supply reacts to this by lowering its output voltage according to its output impedance. However, since this output impedance is generally capacitive, a suddenly appearing large input signal will be amplified at a high clipping level for a short time and then will progress to a lower clipping level. This effect is part of the punch of the amplifier and makes the amplifier sound less compressed.
There are digital devices currently available for producing audio effects such as chorus, flanging, reverberation, vibrato, sampling, pitch change, etc. The delay effects, such as flanging, reverberation and sampling, simply record the signal and play it back later. Pitch change records the signal and plays it back at a different rate. Harmonic analysis of these effects show that all extra frequencies that are generated are created by sampling. None of these effects intentionally introduce harmonics of the signal into the signal.
Another view of these effects is that their basic intent is to recombine a signal that has been delayed and may have been attenuated with itself. Thus, the only harmonics that can be generated are due to the sampling process.
Thus, the primary object of the invention is to provide a semiconductor amplifier which simulates the distortion of a vacuum tube amplifier.
Another object of the invention is to simulate the effects of grid current flowing. This produces even harmonic distortion which is a more pleasant and musical distortion than one made of solely odd harmonics. Further, the grid current effects in a capacitively coupled circuit produces the desirable attach on a note.
A further object of the invention is a guitar amplifier effects preamplifier which may be elegantly professional or may be a simple effects pedal. This is quite possible from the teachings herein because the tube simulation may be done either at low or at high power levels.
A still further objection of the invention is to achieve the general improvement of guitar amplifiers of all types to provide the high power distortion effect at all power levels.
An even further object of the invention is to provide a general use power amplifier which graciously handles excessive inputs.
Another object of the invention is to provide an amplifier with the correct input and output impedances to load the guitar or other source correctly and to drive the speaker correctly.
Another object of the invention is to provide an amplifier with a non-constant or variable gain stage to emulate the distortion of a Class B or AB output stage.
A still further object of the invention is to provide control of the output level of an amplifier as a function of the output level to emulate the effect of the power supply impedance.
An even further object of the invention is to provide an amplifier with a combination bias shifter and distortion stage that operates effectively at low input levels.
A still further object of the invention is to provide mathematical modeling of an amplifier so that the amplifier effect may be created via a digital signal processor.
An even further object of the invention is to provide analog and digital signal processors which intentionally introduces harmonics of the signal into the signal.
A still further object of the invention is the structure of the analog amplifier or the order of calculations of the digital signal processor.
These and other objects of the invention are attained by a distortion synthesizer, having a first distortion circuit, a tone control circuit for altering the tonality of the first distorted signal and a second distorted circuit for introducing harmonics into said tone controlled signal and for clipping the resultant signal as a function of said resultant signal. The system may be analog or a programmed digital processor. The first distortion circuit limits and introduces even harmonics by an asymmetrical bias shifting circuit. The first distortion circuit can also compress the signal. The second distortion circuit includes a variable gain stage which increases with increased input using a plurality of switches which progressively add parallel resistors to progressively increase output current. The clipping circuit in the second distortion circuit clips as a function of amplitude, time or frequency content of the input signal. A direct equalization circuit is provided which emulates a speaker audibly driving a microphone and includes a filter, delay and mixer.
The distortion system provides a distorted signal whose harmonic content increases with increasing input signal. The even harmonics content is initially increased with each cycle of an input signal. Also, the gain and total harmonic distortion increase for a first range of input signal amplitudes, and the gain decreases, and total harmonic distortion increases for a second range of input signal amplitude. In addition, the amplitude of the distorted signal initially decrease and subsequently increases the amplitude of the distorted signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.